The present invention relates to a rear-impact airbag apparatus having an airbag that is inflated and deployed to protect an occupant in response to a rear impact applied to a vehicle.
Like other types of airbag apparatus, the rear-impact airbag apparatus has an inflator and an airbag. The airbag of the rear-impact airbag apparatus is folded and accommodated in an accommodation portion formed in a rear part of the roof. When an impact is applied to the vehicle from the rear, the inflator is activated and ejects gas. Being supplied with gas from the inflator, the airbag is inflated and deployed along the rear window from the accommodation portion. The inflated and deployed airbag protects an occupant seated on the rear seat from the rear impact applied to the vehicle.
A typical rear-impact airbag apparatus is disclosed, for example, in Japanese Laid-Open Patent Publication No. 2004-142481. The rear-impact airbag apparatus of the publication has a lateral inflatable cell and a pair of left and right vertical inflatable cells, which are defined in the airbag. The lateral inflatable cell guides gas to flow outward from an inflator along the vehicle width direction. The vertical inflatable cells each guide gas flowing therein to flow substantially downward from the corresponding end of the lateral inflatable cell. Additional vertical inflatable cells are defined in the airbag. In the inflated and deployed airbag, the additional vertical inflatable cells are located below the lateral inflatable cell and between the left and right vertical inflatable cells. Each inflatable cell is inflated in such a manner that its upper portion has a greater thickness than its lower portion. This configuration effectively improves the occupant protecting performance from a rear impact applied to the vehicle without significantly increasing the inflation amount of the entire airbag.
In the rear-impact airbag apparatus of the above publication, gas that is ejected from the inflator in response to a rear impact applied to the vehicle first flows into the lateral inflatable cell. Then, some of the gas passes through the lateral inflatable cell and flows into the left and right vertical inflatable cells. That is, part of the gas from inflator is guided outward in the vehicle width direction by the lateral inflatable cell, and thereafter, guided substantially downward by the left and right vertical inflatable cells. Thus, the direction of the gas flow is greatly changed at the boundary between the lateral inflatable cell and each of the left and right vertical inflatable cells. Due to an increased flow resistance of the gas, gas cannot smoothly flow through the positions where such a great change in the gas flowing direction takes place. If a smooth gas flow is hampered in this manner, the inflation and deployment of part of the airbag can be delayed, resulting in an uneven inflation and deployment of the airbag.